A scheme called One Point Copy (OPC) is one of the known methods of backing up data stored in a copy-source volume of a product such as a storage device or a computer. OPC creates data called Snapshot, which contains data to be backed up at a certain time point but which does not contain change after the time point. In other words, OPC is a scheme that backs up the entire data in a copy-source volume to be copied at the time point of the creation of the Snapshot.
For example, if a storage system that executes OPC receives an OPC instruction to execute OPC, the system copies the entire data of a copy-source volume at the time of receiving the OPC instruction.
Then, the storage system stores data that is a copy of the data in the copy-source volume, as the Snapshot at the time of receiving the OPC instruction, into a copy-destination volume. In other words, the copy of the data in the copy-source volume can also be regarded as backup data.
For example, the storage system stores the backup data, which is generated by duplicating the entire copy-source volume, into a copy-destination volume in association with session data containing the time of receiving the OPC instruction and the generation of the data being copied.    Patent Literature 1: Japanese Laid-open Patent Publication No. 2007-334709
However, in the event of issuing an overwrite request for a copy-source volume before a Snapshot of the copy-source volume is created, such a conventional data backup manner in a storage system has a possibility that backup of data before the overwriting is not created.
Accompanying drawing FIG. 12 is a diagram depicting a conventional OPC process. In the example of FIG. 12, the storage system includes volumes 01-04, and 11-14. A first copy session stores a Snapshot of the volume 01 into the volume 11; similarly, a second copy session stores a Snapshot of the volume 02 into the volume 12; a third copy session stores a Snapshot of the volume 03 into the volume 13; and a fourth copy session stores a Snapshot of the volume 04 into the volume 14.
In the example of FIG. 12, an writing request is issued to overwrite data “a” into a region of the volume 01 which region stores data A. Similarly, overwrite requests are issued to overwrite data “b” into a region of the volume 02 which region stores data B; overwrite data “c” into a region of the volume 03 which region stores data C; and overwrite data “d” into a region of the volume 04 which region stores data D.
Snapshots of the respective volumes are created in sequence of the volumes 01, 02, 03, and 04. The example of FIG. 12 assumes that a request (I/O processing request) to write the data “c” into the region of data C in the volume 03 is issued at the timing just before creation of a Snapshot of volume 02.
To begin with, a non-illustrated host device issues a Start command of OPC to the volume 01, and responsively, a Snapshot of the region storing the data A in the volume 01 is stored into the volume 11 as depicted by Arrow A1. After that, the data “a” is written into the region storing the data A in the volume 01 at an I/O processing request (see Arrow A2).
Then, the data “c” is written into the region storing the data C in the volume 03 (see Arrow A3). After that, a Start command is issued to the volume 02, and as depicted by Arrow A4, a Snapshot of the region storing the data B in the volume 02 is stored into the volume 12. Furthermore, the data “b” is written into the region storing the data B in the volume 02 (see Arrow A5).
Then, a Start command is issued to the volume 03, and as depicted by Arrow A6, a Snapshot of the volume 03 is stored into the volume 13. Here, since the data C in the volume 03 is already overwritten with the data “c”, a Snapshot of data “c” is stored into the volume 13.
As the above, although the storage system intends to create Snapshots of the data A, B, C, and D stored in the volumes 01-04, a Snapshot of the data “c” after the rewriting is created for the volume 03 because a process at the I/O processing request is carried out before a Start command is issued to the volume 03. Consequently, it is difficult to create Snapshots of multiple volumes ensuring integrity of timing of the Snapshots.
As one solution to create Snapshots of multiple volumes ensuring integrity of the timing of Snapshots, it may be proposed that receipt of an I/O processing request is temporarily stopped until processing related to Start commands for all the volumes 01-04 is completed to prevent an I/O process from being executed before a Start command is issued.
However, temporarily stopping receipt of an I/O processing request until processing related to Start commands for all the volumes 01-04 is completed may have a possibility of delaying I/O processes and affecting the operation of the storage device.